Polyphenylene ether resins are high performance engineering thermoplastics having relatively high melt viscosities and softening points (i.e., in excess of 250.degree. C.). They are useful for many commercial applications requiring high temperature resistance and can be formed into films, fibers and molded articles. For example, they can be used to form a wide range of products including household applicances, automotive parts and trim. The polyphenylene ether resins and methods of their preparation have been thoroughly described in the patent literature, including Hay, U.S. Pat. Nos. 3,306,874 and 3,306,875 and Stamatoff, U.S. Pat. Nos. 3,257,357 and 3,257,358, and elsewhere.
As is well established, the polyphenylene ether resins may be utilized alone or in admixture with styrene polymers over a broad spectrum of proportions. The resulting blends can be molded into many of the same articles made from polyphenylene ether resins alone, but with the advantage that the moldings often possess better physical and/or chemical properties. Compositions of polyphenylene ether resins and styrene polymers are also described in the patent literature, including Cizek, U.S. Pat. No. 3,383,435, and other patents known to those skilled in the art.
Compositions of polyphenylene ether resin and styrene polymers are not normally flame retardant, and there are instances when it is desirable to impart a degree of flame retardancy to the compositions such that the molded articles are better able to resist burning or melting when exposed to elevated temperatures or placed near an open flame. To this end, it has been disclosed in the art that certain halogenated compounds, and brominated compounds in particular, are effective as flame retardant additives for polyphenylene ether-polystyrene blends. An example of such a disclosure is Reinhard, U.S. Pat. No. 3,809,729.
Foamable compositions of polyphenylene ether resins with or without styrene polymers are particularly suited as sources of lightweight structural substitutes for metals, especially in the automotive industry.
Foaming agents are customarily employed to make moldable compositions including compositions containing polyphenylene ether resins. Foaming agents tend to reduce the weight of a thermoplastic composition to achieve light weight properties by decomposing under reaction conditions with the evolution of gas. Blowing agents have been used to obtain foamable compositions of a variety of thermoplastic materials as described in, for example, Muller et al., U.S. Pat. No. 3,781,233 and Fox, U.S. Pat. No. 4,280,005 and the United States and British patent references cited therein. Lundberg, U.S. Pat. No. 3,947,387 and Liu, U.S. Pat No. 4,588,754 disclose foamed polymeric products using citric acid, alone, or citric acid mixed with sodium bicarbonate as foaming agents.
Halogenated flame retardants (e.g. brominated styrene) are preferred over phosphate based flame retardants because the former exhibit less stress cracking. However, previous attempts to incorporate halogenated flame retardants into polyphenylene ether/polystyrene resins and then to foam such compositions have not been successful becase the compositions tend to degrade, often giving streaked articles, and generating odors, despite efforts to use a variety of foaming agents. This is especially a problem when attempting to produce large sized articles by foaming the high shot weights (e.g., 14-16 oz. per part) and low shot times (e.g., 0.2 to 0.3 seconds per part).
Applicants have now discovered that a foaming agent comprising a combination of citric acid and sodium bicarbonate provides an effective means of making moldable compositions derived from polyphenylene ether resins, styrene polymers and halogenated flame retardants (e.g., brominated styrenes) without the evolution of undesirable by-products which lead to the decomposition of the moldable composition (e.g. surface streaking and odor generation). Moreover, the composition is uniquely suitable to produce large size foamed, shaped articles free of streaking and odor generation.
It is therefore an object of the invention to provide moldable, foamable flame retardant polyphenylene ether compositions with reduced surface streaking and odor generation.
It is still a further object of the invention to provide methods of making and using such flame retardant polyphenylene ether compositions.
It is a principal object of the present invention to produce large sized articles by foaming flame retardant polyphenylene ether resin compositions using high shot weights and short shot times.